Towards the Quantum Internet! Diamond NV Color Center Emits High-Quality Single Photons

Diamond (diamond) materials are important for future technologies such as the quantum Internet: special defective centers can be used as quantum bits and emit single light particles called single photons.

However, in order to transmit data at a viable communication rate in a quantum network, all photons must be collected in an optical fiber and not lost during transmission; it must also be ensured that they all have the same color (i.e., the same frequency).

Until now, meeting these requirements has been impossible.

To overcome this limitation, members of the Integrated Quantum Photonics group led by Prof. Tim Schröder at Humboldt University Berlin have succeeded for the first time in the world in generating and detecting photons with stable photon frequencies emitted from a "quantum light source" (more precisely, from the center of a nitrogen vacancy defect in a diamond nanostructure).

The results were published in Physical Review X under the title "Optically Coherent Nitrogen-Vacancy Defect Centers in Diamond Nanostructures".

By carefully selecting diamond materials, the experimental team performed complex nanofabrication experiments at the Ferdinand Braun Joint Laboratory of the Leibniz Institute for High Frequency Technology (FBH). By combining these methods, the electronic noise that previously interfered with data transmission can be significantly reduced, while photons are emitted at a stable (communication) frequency.

Schematic of NV incorporated into a nanopillar.

Furthermore, the Berlin researchers have shown that with the developed method, the communication rate between currently spatially separated quantum systems can be expected to increase by more than 1,000 times - an important step toward the quantum Internet of the future.

Experimental results on entanglement generation

Today, scientists have integrated individual quantum bits into optimized diamond nanostructures. These structures are 1,000 times thinner than a human hair, making it possible to transfer emitted photons into glass fibers in a targeted manner.

However, during the fabrication of the nanostructures, the material surface is damaged at the atomic level and the free electrons create uncontrollable noise for the resulting light particles. The noise is comparable to unstable radio frequencies, leading to fluctuations in photon frequencies that can prevent some quantum operations (e.g., entanglement).

A characteristic of the diamond material used is the relatively high atomic density of nitrogen impurities in its lattice, and these may shield the quantum light source from electronic noise at the surface of the nanostructure.

The centers of defects in the diamond nanostructures can be used as quantum bits. Through quantum manipulation (entanglement), quantum information could be stored in emitted single photons and transmitted over optical fibers in a future quantum Internet.

In this experiment, the team demonstrated a device based on nanostructured coupled NV with optical properties suitable for quantum coherent control protocols. However, more exact physical processes need to be studied in more detail in the future.

Reference links:

[1] Phys. Rev. X 13, 011042 (2023) - Optically Coherent Nitrogen-Vacancy Defect Centers in Diamond Nanostructures (aps.org)

[2] https://phys.org/news/2023-04-important-quantum-internet-diamond-nanostructures.html